1. Field of the Invention
The present invention relates to a heat storage material of NaCH.sub.3 COO.3H.sub.2 O or a mixture thereof which utilizes the latent heat of fusion. The heat storage material is used for air-conditioning, waste heat recovery or solar heat storage.
2. Description of the Prior Art
As a heat storage material in a range of 50.degree.-60.degree. C. which utilizes a latent heat of fusion, a hypo (Na.sub.2 S.sub.2 O.sub.3.5H.sub.2 O; melting point of 48.degree. C.; heat of fusion of 50 cal/g) has been proposed. However, it has not been recently considered to be important because of an easy solidification of Na.sub.2 S.sub.2 O.sub.3.2H.sub.2 O which causes inferior phase change stability and no practical development of a nucleating method.
On the other hand, NaCH.sub.3 COO.3H.sub.2 O has an excellent phase change stability higher than that of hypo. Thus, NaCH.sub.3 COO.3H.sub.2 O has been recently considered to be important. However, NaCH.sub.3 COO.3H.sub.2 O (melting point of 58.degree. C.; heat of fusion of 60 cal/g.) is as remarkably super-coolable as hypo. Therefore, in order to utilize the latent heat of fusion, it is most important to prevent the supercooling. It has been known to prevent the supercooling by adding water to NaCH.sub.3 COO.3H.sub.2 O i.e. the prior invention disclosed in Japanese Unexamined Patent Publication 126980/1976. Although the supercooled state is easily broken by the addition of water, reliability for a long time is not satisfactory. The nucleating agent such as Na.sub.2 (CH.sub.2 COO).sub.2.6H.sub.2 O, i.e. the prior invention disclosed in Japanese Unexamined Patent Publication 16035/1980, does not impart the nucleating effect unless cooled to 30.degree. C. or lower whereby there is the disadvantage or narrowly limited applications.
The mixture of NaCH.sub.3 COO.3H.sub.2 O and another salt, such as NaBr.2H.sub.2 O, at a content of 10 wt. %, i.e. the prior invention disclosed in Japanese Unexamined Patent Publication 14173/1978, allows repeated smooth phase changes for more than 400 times in a heat cycle test of 60.degree. C.-40.degree. C. (8 cycles per day). Although, the mixture thereof containing NaHCOO.3H.sub.2 O at a content of 16 wt. % can have the self-nucleating effect, the reliability is not disadvantageously high.
As described, the nucleating method for the heat storage material made of NaCH.sub.3 COO.3H.sub.2 O or a mixture of NaCH.sub.3 COO.3H.sub.2 O is partially successful, however, it is not practically successful. There is no report on the nucleating agent for NaCH.sub.3 COO.3H.sub.2 O other than the inventors' prior disclosures. In accordance with the method of U.S. Pat. No. 4,077,390, the metal piece in the liquid must be externally bent at the nucleation for each time. Thus, the method is not practically employed.